sdBs

Subdwarf B stars (sdB or sdBs in plural) are hot, compact stars characterized by high temperatures and surface gravities and helium-core burning with very thin hydrogen envelopes. They typically have effective temperatures of 20,000–40,000 K, surface gravities log g ≈ 5.0–6.0, masses near 0.45–0.5 solar masses, and radii around 0.15–0.25 solar radii. Their spectra show strong Balmer lines and helium lines; helium-rich sdBs exist but are less common.

Formation and evolution: They are located on the extreme horizontal branch, having ignited helium in the core after the envelope was stripped. Most sdBs are products of binary evolution, with channels including common-envelope ejection producing short-period binaries, stable Roche lobe overflow forming wider binaries, and mergers of two helium-core white dwarfs. The binary fraction is high; companions can be white dwarfs, low-mass main-sequence stars, or substellar objects; many systems have orbital periods from hours to days.

Pulsations and asteroseismology: A subset are pulsating variables. Short-period p-mode sdB stars (V361 Hya or EC 14026) show periods of a few minutes; long-period g-mode sdB stars (V1093 Her) have hours-long periods; there are hybrid pulsators. Asteroseismic studies constrain envelope mass, core structure, and rotation.

Occurrences and significance: They are found primarily in the Milky Way field and, less commonly, in globular clusters. They contribute to the ultraviolet upturn observed in early-type galaxies and are important for tests of binary stellar evolution and population synthesis models. Distinguishing sdBs from other hot subdwarfs (sdO) is based on temperature, gravity, and spectral features.